An ocean thermal energy conversion (OTEC), now undergoing development, is a less-massive, more-efficient means of exploiting the same basic principle as that of the proposed system described in “Alternative OTEC Scheme for a Submarine Robot” (NPO-43500), NASA Tech Briefs, Vol. 33, No. 1 (January 2009), page 50. The proposed system as described previously would be based on the thawing-expansion/freezing-contraction behavior of a wax or perhaps another suitable phase-change material (PCM). The power generated by the system would be used to recharge the batteries in a battery-powered unmanned underwater vehicle [UUV (essentially, a small exploratory submarine robot)] of a type that has been deployed in large numbers in research pertaining to global warming. A UUV of this type travels between the ocean surface and depths, measuring temperature and salinity.

The PCM Expands and Contracts upon melting near the ocean surface and freezing at depth, respec- tively. The expansion and contraction causes the hydraulic fluid to flow cyclically through the hydraulic motor to drive the alternator to charge the battery.
At one phase of its operational cycle, the previously proposed system would utilize the surface ocean temperature (which lies between 15 and 30 °C over most of the Earth) to melt a PCM that has a melting/freezing temperature of about 10 °C. At the opposite phase of its operational cycle, the system would utilize the lower ocean temperature at depth (e.g., between 4 and 7 °C at a depth of 300 m) to freeze the PCM. The melting or freezing would cause the PCM to expand or contract, respectively, by about 9 volume percent. The PCM would be contained in tubes that would be capable of expanding and contracting with the PCM. The PCM-containing tubes would be immersed in a hydraulic fluid. The expansion and contraction would drive a flow of the hydraulic fluid against a piston that, in turn, would push a rack-and-pinion gear system to spin a generator to charge a battery.

The present system was conceived because the previously proposed system was found to be too heavy and inefficient for the intended application. The main difference between the present and previously proposed systems is that in the present system, the flow of hydraulic fluid drives a hydraulic motor instead of a piston.

The operational cycle of the present system involves three phases of flow of the hydraulic fluid and is best understood by reference to the figure. In phase 1, near the ocean surface, valve 1 is held open and valves 2 and 3 are held closed, and the expansion of PCM upon heating to >10 °C pushes the hydraulic fluid through valve 1 into the bellows in a gas-spring/bellows chamber, charging the chamber to an absolute pressure of about 3 kpsi (≈21 MPa). In phase 2, valve 2 is opened, allowing the pressurized hydraulic fluid to flow through the hydraulic motor and into the bellows in a second, lower-pressure gas-spring/bellows chamber. Upon completion of this flow, valves 1 and 2 are closed and valve 3 opened in anticipation of phase 3. In phase 3, which takes place upon cooling to

This work was done by Yi Chao, Jack Jones, and Thomas Valdez of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management JPL Mail Stop 202-233 4800 Oak Grove Drive
Pasadena, CA 91109-8099 E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-45404, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Cold weather Engine components Engine mechanical components Engines Fluid power systems Hydraulic fluids Hydraulic motors Hydraulic systems Parts Physical Sciences Pistons Powertrains Research and development Unmanned Underwater and Surface Vehicles (UUW/USV) Valves Weather and climate
This Brief includes a Technical Support Package (TSP).
Document cover
Improved OTEC System for a Submarine Robot

(reference NPO-45404) is currently available for download from the TSP library.

Don't have an account?

Magazine cover
NASA Tech Briefs Magazine

This article first appeared in the May, 2010 issue of NASA Tech Briefs Magazine (Vol. 34 No. 5).

Read more articles from this issue here.

Read more articles from the archives here.

Overview

The document titled "Improved OTEC System for a Submarine Robot" (NPO 45404) from NASA's Jet Propulsion Laboratory outlines an innovative approach to harnessing ocean thermal energy for powering submersible robots. The primary challenge addressed is the need for a reliable power source to recharge batteries for long-duration underwater missions, utilizing the temperature differences between warmer surface waters and cooler deep waters.

The previous solution, referenced in NTR#43500, involved using phase-change materials (PCM) that melted in warmer ocean temperatures (15-25°C) and froze in cooler temperatures (4-7°C). This phase change generated pressure in a secondary liquid, which was used to drive a piston connected to a generator. However, this method was deemed too heavy and inefficient by the Office of Naval Research.

The new solution proposed in this document involves a more efficient design that utilizes a hydraulic motor instead of a piston. The high-pressure liquid generated by the PCM flows through the hydraulic motor, which is connected to an alternator. This alternator generates alternating current (AC) that is then converted to direct current (DC) for battery storage. This design is noted for being simpler, lighter, and more efficient than the previous method.

One of the significant advantages of this system is its ability to vent low-pressure fluid to an external bladder at depths such as 300 meters (450 psi). This capability allows for buoyancy control, where 15% of the energy generated is used for buoyancy, enabling the submersible to ascend to the surface, while the remaining 85% is utilized for power generation.

The document emphasizes the potential applications of this technology beyond submersibles, highlighting its relevance in broader technological, scientific, and commercial contexts. It is part of NASA's Commercial Technology Program, aimed at making aerospace-related developments accessible for wider use.

For further inquiries or detailed information, the document provides contact details for the Innovative Technology Assets Management at JPL. Overall, this technical support package showcases a significant advancement in underwater energy generation, promising enhanced operational capabilities for submersible robots.